The present invention relates generally to a method for increasing the efficiency of a vapor compression system by heating the refrigerant in the evaporator with heat provided by the compressor.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. xe2x80x9cNaturalxe2x80x9d refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide to run transcritical, or above the critical point.
When a vapor compression system runs transcritical, the high side pressure of the refrigerant is typically high so that the refrigerant does not change phases from vapor to liquid while passing through the heat rejecting heat exchanger. Therefore, the heat rejecting heat exchanger operates as a gas cooler in a transcritical cycle, rather than as a condenser. The pressure of a subcritical fluid is a function of temperature under saturated conditions (where both liquid and vapor are present). However, the pressure of a transcritical fluid is a function of fluid density when the temperature is higher than the critical temperature.
In a prior vapor compression system, the heat generated by the compressor motor either is lost by being discharged to the ambient or superheats the suction gas in the compressor. If the heat superheats the suction gas in the compressor, the density and the mass flow rate of the refrigerant decreases, decreasing system efficiency. It would be beneficial to utilize compressor heat to improve system efficiency and reduce system size and cost.
The efficiency of a vapor compression system can be increased by coupling the evaporator with the compressor to provide heat from the compressor to the refrigerant in the evaporator. An intercooler of a two-stage vapor compression system or a compressor component can also be coupled to the evaporator to provide the heat to the evaporator refrigerant. Preferably, the compressor component is a compressor oil cooler or a compressor motor. The refrigerant in the evaporator accepts heat from the refrigerant in the intercooler or the compressor component, increasing the temperature of the refrigerant in the evaporator. As pressure is directly related to temperature, the temperature of the refrigerant in the evaporator increases, increasing the low side pressure of the refrigerant exiting the evaporator. As the low side pressure increases, the compressor needs to do less work to bring the refrigerant to the high side pressure, increasing system efficiency and/or capacity.
Additionally, as the heat from the refrigerant in the intercooler or the compressor component is rejected to the refrigerant in the evaporator, the refrigerant in the compressor is cooled. By cooling the refrigerant in the compressor, the density and the mass flow rate of the refrigerant in the compressor increases, increasing system efficiency.
These and other features of the present invention will be best understood from the following specification and drawings.